The present invention relates, in general, to signal processing circuits and, more particularly, to switched capacitor circuits.
Switched capacitor circuits are widely used in signal processing applications such as, for example, filtering, analog to digital converting, digital to analog converting, or the like. An effect that degrades the performance of a switched capacitor circuit is the sampling noise. Using small capacitors in a switched capacitor circuit improves the power efficiency and high frequency performance of the switched capacitor circuit. However, the magnitude of the sampling noise is substantially proportional to the temperature and inversely proportional to the capacitance values of the sampling capacitors in the switched capacitor circuit. Therefore, small capacitors exhibit large sampling noise.
The sampling noise can be reduced by sampling an input signal at a higher frequency than that of the input signal, i.e., oversampling the input signal in the time domain. For example, the input signal can be sampled at a frequency higher than the Nyquist frequency of the input signal by a factor of two, three, four, five, etc. The ratio of the sampling frequency to the Nyquist frequency of the input signal is conventionally referred to as the sampling rate (.times.). When the sampling rate is greater than one, the sampling is referred to as oversampling. For a bandwidth limited signal, the sampling noise is substantially inversely proportional to the sampling rate. Thus, sampling an input signal at a rate of four, i.e., 4.times.oversampling, reduces the sampling noise approximately by a factor of four compared with sampling the input signal a rate of one, i.e., 1.times.sampling. However, oversampling in the time domain increase the power consumption of the circuit. Further, the sampling rate has an upper limit determined by the high frequency characteristics of the devices in the circuit.
Accordingly, it would be advantageous to have a switched capacitor circuit and a method for reducing sampling noise therein. It is desirable for the circuit to be simple. It is also desirable for the circuit and the method to be power efficient. It would be of further advantage for the circuit and the method to be suitable for use in high frequency applications.